Martensitic stainless steel seamless pipe for oil country tubular goods, and method for producing same

ABSTRACT

Provided herein is a martensitic stainless steel seamless pipe, intended for oil country tubular goods, having high strength, and excellent sulfide stress corrosion cracking resistance. A method for producing such a martensitic stainless steel seamless pipe is also provided. The martensitic stainless steel seamless pipe for oil country tubular goods has a composition that contains, in mass %, C: 0.035% or less, Si: 0.5% or less, Mn: 0.05 to 0.5%, P: 0.03% or less, S: 0.005% or less, Cu: 2.6% or less, Ni: 5.3 to 7.3%, Cr: 11.8 to 14.5%, Al: 0.1% or less, Mo: 1.8 to 3.0%, V: 0.2% or less, N: 0.1% or less, and the balance Fe and unavoidable impurities, and in which C, Mn, Cr, Cu, Ni, Mo, W, Nb, N, and Ti satisfy the predetermined relations.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2017/033008, filed Sep. 13, 2017, which claims priority to Japanese Patent Application No. 2016-208420, filed Oct. 25, 2016, the disclosures of these applications being incorporated herein by reference in their entireties for all purposes.

FIELD OF THE INVENTION

The present invention relates to a martensitic stainless steel seamless pipe for use in oil country tubular goods used in oil well and gas well applications such as in crude oil wells and natural gas wells, and to a method for producing such a martensitic stainless steel seamless pipe. The invention particularly relates to improvement of corrosion resistance in a severe corrosive environment containing carbon dioxide gas (CO₂), chlorine ions (Cl⁻), and the like, and improvement of sulfide stress corrosion cracking resistance (SSC resistance) in a hydrogen sulfide (H₂S)-containing environment.

BACKGROUND OF THE INVENTION

Rising crude oil prices, and the increasing shortage of petroleum resources have prompted active development of deep oil fields that were unthinkable in the past, and oil fields and gas fields of a severe corrosive environment containing carbon dioxide gas, chlorine ions, and hydrogen sulfide. Steel pipes for oil country tubular goods (OCTG) intended for such an environment need to be made of materials having high strength, and excellent corrosion resistance.

Oil country tubular goods used for mining of oil fields and gas fields of an environment containing CO₂ gas, Cl⁻, and the like often use 13% Cr martensitic stainless steel pipes. In order to meet the increasing demand for higher SSC resistance arising out of the world-wide development of oil fields of a severe corrosive environment containing hydrogen sulfide, there has been increasing use of modified 13% Cr martensitic stainless steel pipes containing a reduced carbon content, and increased Ni and Mo contents.

PTL 1 discloses a 13% Cr steel basic composition containing Ni, Mo, and Cu, and a much smaller carbon content than in traditional compositions. These elements are contained to satisfy Cr+2Ni+1.1Mo+0.7Cu≤32.5, and Nb+V≥0.05% for at least one of Nb: 0.20% or less, and V: 0.20% or less. The composition is described as being capable of providing high strength with a yield strength of 965 MPa or more, and high toughness with a Charpy absorption energy at −40° C. of 50 J or more, in addition to desirable corrosion resistance.

PTL 2 describes a 13% Cr martensitic stainless steel pipe having an extremely low carbon content of 0.015% or less, and a Ti content of 0.03% or more. With such a composition, the 13% Cr martensitic stainless steel pipe can have high strength with a yield stress in the order of 95 ksi (655 to 758 MPa), low hardness with a Rockwell hardness HRC of less than 27, and excellent SSC resistance. PTL 3 describes a martensitic stainless steel that satisfies 6.0≤Ti/C≤10.1, where Ti/C has a correlation with a value obtained by subtracting the yield stress from the tensile stress. The technique described in this publication can produce a value of 20.7 MPa or more as the difference of the yield stress from the tensile stress, and can reduce the hardness variation, which deteriorates the SSC resistance.

PTL 4 describes a martensitic stainless steel containing a specified amount of molybdenum satisfying Mo≥2.3−0.89Si+32.2C, and having a metal structure that is configured primarily from tempered martensite, carbides that have precipitated during tempering, and intermetallic compounds, such as the Laves phase and the δ phase, that have finely precipitated during tempering. The technique described in this publication can achieve high strength with a 0.2% proof stress of 860 MPa or more, and excellent carbon dioxide corrosion resistance, and excellent sulfide stress corrosion cracking resistance.

PATENT LITERATURE PTL 1: JP-A-2007-332442 PTL 2: JP-A-2010-242163 PTL 3: WO2008/023702 PTL 4: WO2004/057050 SUMMARY OF THE INVENTION

Recent oil fields and gas fields are developed in severe corrosive environments containing CO₂, Cl⁻, and H₂S. There are also rising concerns over increased H₂S concentrations due to aging. The oil country tubular goods used in these environments are thus required to have excellent sulfide stress corrosion cracking resistance (SSC resistance), in addition to carbon dioxide corrosion resistance. The technique of PTL 1 is described as providing excellent carbon dioxide corrosion resistance. However, there is no investigation of sulfide stress corrosion cracking resistance, and the technique cannot be said as providing the level of corrosion resistance that can withstand a severe corrosive environment.

It is stated in PTL 2 that sulfide stress corrosion cracking resistance can be maintained under an applied stress of 655 MPa in an atmosphere of a 5% NaCl aqueous solution (H₂S: 0.10 bar) with an adjusted pH of 3.5. PTL 3 describes providing sulfide stress corrosion cracking resistance in an atmosphere of a 20% NaCl aqueous solution (H₂S: 0.03 atm, 002 balance.) with an adjusted pH of 4.5. PTL 4 describes providing sulfide stress corrosion cracking resistance in an atmosphere of a 25% NaCl aqueous solution (H₂S: 0.003 MPa, 002 balance.) with an adjusted pH of 4.0. However, these techniques do not investigate sulfide stress corrosion cracking resistance in other atmospheres, and cannot be said as providing the level of sulfide stress corrosion cracking resistance that can withstand the today's more severe corrosive environments.

It is accordingly an object according to aspects of the present invention to provide a martensitic stainless steel seamless pipe having high strength, and excellent sulfide stress corrosion cracking resistance, intended for oil country tubular goods. Aspects of the present invention are also intended to provide a method for producing such a martensitic stainless steel seamless pipe for oil country tubular goods.

As used herein, “high-strength” means a yield stress of 758 MPa (110 ksi) or more. Preferably, the yield stress is 896 MPa or less.

As used herein, “excellent sulfide stress corrosion cracking resistance” means that a test piece dipped in a test solution (a 0.165 mass % NaCl aqueous solution; liquid temperature: 25° C., H₂S: 1 bar, CO₂ balance) having an adjusted pH of 3.5 with addition of sodium acetate and hydrochloric acid does not crack even after 720 hours under an applied stress equal to 90% of the yield stress.

In order to achieve the foregoing objects, the present inventors conducted intensive studies of various alloy elements in a basic composition of a 13% Cr stainless steel pipe with regard to the effects of these elements on sulfide stress corrosion cracking resistance (SSC resistance) in a corrosive environment containing CO₂, Cl⁻, and H₂S. As a result of the investigation, the present inventors have found that a martensitic stainless steel seamless pipe for oil country tubular goods having the desired strength, and excellent SSC resistance in a CO₂—, Cl⁻—, and H₂S-containing corrosive environment under an applied stress close to the yield stress can be produced when a composition containing C, Mn, Cr, Cu, Ni, Mo, W, Nb, N, and Ti in adjusted amounts that satisfy appropriate relations and ranges is subjected to appropriate quenching and tempering treatments.

Aspects of the present invention were completed on the basis of these findings after further studies, and are as follows.

[1] A martensitic stainless steel seamless pipe for oil country tubular goods,

the martensitic stainless steel seamless pipe having a composition that comprises, in mass %, C: 0.035% or less, Si: 0.5% or less, Mn: 0.05 to 0.5%, P: 0.03% or less, S: 0.005% or less, Cu: 2.6% or less, Ni: 5.3 to 7.3%, Cr: 11.8 to 14.5%, Al: 0.1% or less, Mo: 1.8 to 3.0%, V: 0.2% or less, N: 0.1% or less, and the balance Fe and unavoidable impurities, and that satisfies the following formula (4) with the following formulae (1), (2), and (3),

the martensitic stainless steel seamless pipe having a yield stress of 758 MPa or more.

−109.37C+7.307Mn+6.399Cr+6.329Cu+11.343Ni−13.529Mo+1.276W+2.925Nb+196.775N−2.621Ti−120.307  Formula (1)

−0.0278Mn+0.0892Cr+0.00567Ni+0.153Mo−0.0219W−1.984N+0.208Ti−1.83  Formula (2)

−1.324C+0.0533Mn+0.0268Cr+0.0893Cu+0.00526Ni+0.0222Mo−0.0132W−0.473N−0.5Ti−0.514,  Formula (3)

In the formulae (1) to (3), C, Mn, Cr, Cu, Ni, Mo, W, Nb, N, and Ti represent the content of each element in mass % (the content being 0 (zero) percent for elements that are not contained).

−10≤formula (1)≤45, −0.25≤formula (2)≤−0.20, and 0.10≤formula (3)≤0.20  Formula (4)

[2] The martensitic stainless steel seamless pipe for oil country tubular goods according to item [1], wherein the composition further comprises, in mass %, at least one selected from Ti: 0.19% or less, Nb: 0.25% or less, W: 1.1% or less, and Co: 0.45% or less.

[3] A′ method for producing the martensitic stainless steel seamless pipe for oil country tubular goods of item [1] or [2], the method comprising:

making a steel pipe out of a steel pipe material;

subjecting the steel pipe to quenching in which the steel pipe is heated to a temperature equal to or greater than the Ac₃ transformation point, and air cooled to a cooling stop temperature of 100° C. or less at a cooling rate of 0.1° C./s or more; and tempering the steel pipe at a temperature equal to or less than the Ac₁ transformation point.

Aspects of the present invention can produce a martensitic stainless steel seamless pipe for oil country tubular goods having excellent sulfide stress corrosion cracking resistance (SSC resistance) in a CO₂—, Cl⁻—, and H₂S-containing corrosive environment, and high strength with a yield stress YS of 758 MPa (110 ksi) or more.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

A seamless stainless steel pipe according to aspects of the present invention is a martensitic stainless steel seamless pipe for oil country tubular goods. The martensitic stainless steel seamless pipe has a composition that contains, in mass %, C: 0.035% or less, Si: 0.5% or less, Mn: 0.05 to 0.5%, P: 0.03% or less, S: 0.005% or less, Cu: 2.6% or less, Ni: 5.3 to 7.3%, Cr: 11.8 to 14.5%, Al: 0.1% or less, Mo: 1.8 to 3.0%, V: 0.2% or less, N: 0.1% or less, and the balance Fe and unavoidable impurities, and that satisfies the following formula (4), (5), or (6) with the following formulae (1), (2), and (3). The martensitic stainless steel seamless pipe has a yield stress of 758 MPa or more.

−109.37C+7.307Mn+6.399Cr+6.329Cu+11.343Ni−13.529Mo+1.276W+2.925Nb+196.775N−2.621Ti−120.307  Formula (1)

−0.0278Mn+0.0892Cr+0.00567Ni+0.153Mo−0.0219W−1.984N+0.208Ti−1.83  Formula (2)

−1.324C+0.0533Mn+0.0268Cr+0.0893Cu+0.00526Ni+0.0222Mo−0.0132W−0.473N−0.5Ti−0.514,  Formula (3)

In the formulae (1) to (3), C, Mn, Cr, Cu, Ni, Mo, W, Nb, N, and Ti represent the content of each element in mass % (the content being 0 (zero) percent for elements that are not contained).

−10≤formula (1)≤45, −0.25≤formula (2)≤−0.20, and 0.10≤formula (3)≤0.20  Formula (4)

−10≤formula (1)≤5, −0.355≤formula (2)≤−0.25, and 0.0255≤formula (3)≤0.10  Formula (5)

−10≤formula (1)≤−5, −0.39≤formula (2)≤−0.35, and −0.15≤formula (3)≤0.025  Formula (6)

The reasons for specifying the composition of the steel pipe according to aspects of the present invention are as follows. In the following, “%” means percent by mass, unless otherwise specifically stated.

C: 0.035% or Less

Carbon is an important element involved in the strength of the martensitic stainless steel, and effectively improves the strength. However, a carbon content of more than 0.035% makes the hardness excessively high, and increases the sensitivity to sulfide stress corrosion cracking. For this reason, the C content is limited to 0.035% or less in accordance with aspects of the present invention. Preferably, the C content is 0.015% or less. More preferably, the C content is 0.0090% or less. Further preferably, the C content is 0.0075% or less. Desirably, carbon is contained in an amount of 0.005% or more to provide the desired strength.

Si: 0.5% or Less

Silicon acts as a deoxidizing agent, and should be contained in an amount of 0.05% or more. A Si content of more than 0.5% deteriorates carbon dioxide corrosion resistance and hot workability. For this reason, the Si content is limited to 0.5% or less. The lower limit of Si content is preferably 0.10% or more, and the upper limit of Si content is preferably 0.30% or less.

Mn: 0.05 to 0.5%

Manganese is an element that improves hot workability, and is contained in an amount of 0.05% or more. When contained in excess of 0.5%, the effect becomes saturated, and this leads to increased cost. For this reason, the Mn content is limited to 0.05 to 0.5%. Preferably, the Mn content is 0.40% or less.

P: 0.03% or Less

Phosphorus is an element that deteriorates carbon dioxide corrosion resistance, pitting corrosion resistance, and sulfide stress corrosion cracking resistance, and should be contained in as small an amount as possible in accordance with aspects of the present invention. However, an excessively small P content leads to increased manufacturing cost. The P content is therefore limited to 0.03% or less, a content that does not bring about an excessive loss of characteristics, and that is industrially feasible in terms of cost. Preferably, the P content is 0.02% or less.

S: 0.005% or Less

Sulfur is an element that seriously deteriorates hot workability, and should desirably be contained in as small an amount as possible. A S content of 0.005% or less enables pipe production using common procedures, and accordingly the S content is limited to 0.005% or less in accordance with aspects of the present invention. Preferably, the S content is 0.003% or less.

Cu: 2.6% or Less

Copper adds strength to the protective coating, and improves sulfide stress corrosion cracking resistance. However, a Cu content of more than 2.6% causes precipitation of CuS, and deteriorates hot workability. For this reason, the Cu content is limited to 2.6% or less. The lower limit of Cu content is preferably 0.5% or more, and the upper limit of Cu content is preferably 2.0% or less.

Ni: 5.3 to 7.3%

When contained in an amount of 5.3% or more, nickel adds strength to the protective coating, and improves corrosion resistance. Nickel also forms a solid solution, and increases the steel strength in this content range. A Ni content of more than 7.3% makes the martensite phase unstable, and the strength deteriorates. For this reason, the Ni content is limited to 5.3 to 7.3%. Preferably, the Ni content is 5.7% or more, more preferably 6.0% or more.

Cr: 11.8 to 14.5%

Chromium is an element that forms a protective coating, and improves the corrosion resistance. Chromium provides the corrosion resistance necessary for oil country tubular goods applications when contained in an amount of 11.8% or more. A Cr content or more than 14.5% facilitates ferrite generation, and the martensite phase cannot remain stable. For this reason, the Cr content is limited to 11.8 to 14.5%. The lower limit of Cr content is preferably 12.0% or more, and the upper limit of Cr content is preferably 13.5% or less.

Al: 0.1% or Less

Aluminum acts as a deoxidizing agent. An Al content of 0.01% or more effectively provides this effect. Because an Al content of more than 0.1% adversely affects toughness, the Al content is limited to 0.1% or less in accordance with aspects of the present invention. Preferably, the Al content is 0.01 to 0.03%.

Mo: 1.8 to 3.0%

Molybdenum is an element that improves the pitting corrosion resistance caused by Cl⁻. Molybdenum needs to be contained in an amount of 1.8% or more to obtain the corrosion resistance necessary for a severe corrosive environment. The effect becomes saturated when the Mo content is more than 3.0%. Molybdenum is also an expensive element, and increases the manufacturing cost. For these reasons, the Mo content is limited to 1.8 to 3.0%. The lower limit of Mo content is preferably 2.4% or more, and the upper limit of Mo content is preferably 2.9% or less.

V: 0.2% or Less.

Vanadium is contained in an amount of desirably 0.01% or more, in order to improve steel strength by precipitation strengthening, and to improve sulfide stress corrosion cracking resistance. A V content of more than 0.2% deteriorates toughness, and the V content is limited to 0.2% or less in accordance with aspects of the present invention. The lower limit of V content is preferably 0.01% or more, and the upper limit of V content is preferably 0.08% or less.

N: 0.1% or Less

Nitrogen is an element that greatly improves the pitting corrosion resistance. However, a N content of more than 0.1% causes formation of various nitrides, and deteriorates toughness. For this reason, the N content is limited to 0.1% or less in accordance with aspects of the present invention. Preferably, the N content is 0.003% or more. The lower limit of N content is more preferably 0.004% or more, further preferably 0.005% or more. The upper limit of N content is more preferably 0.08% or less, further preferably 0.05% or less.

In accordance with aspects of the present invention, C, Mn, Cr, Cu, Ni, Mo, W, Nb, N, and Ti are contained in the foregoing ranges, and these are contained so as to satisfy the formula (4), (5), or (6) with the formulae (1), (2), and (3) below. Formula (1) is a formula that correlates with the residual γ amount. By making the calculated value of formula (1) smaller, the residual austenite is reduced, and the hardness reduces, with the result that the sulfide stress corrosion cracking resistance improves. Formula (2) is a formula that correlates with the repassivation potential. Regeneration of the passivation coating occurs more easily, and repassivation improves by containing C, Mn, Cr, Cu, N Mo, W, Nb, N, and Ti in such amounts that formula (1) yields a value that satisfies the range of formula (4), (5), or (6), and by containing Mn, Cr, Cu, Ni, Mo, W, N, and Ti in such amounts that formula (2) yields a value that satisfies the range of formula (4), (5), or (6). Formula (3) is a formula that correlates with the pitting corrosion potential. Pitting corrosion, which becomes an origin of sulfide stress corrosion cracking, can be suppressed, and the sulfide stress corrosion cracking resistance greatly improves by containing C, Mn, Cr, Cu, Ni, Mo, W, N, and Ti in such amounts that formula (3) yields a value that satisfies the range of formula (4), (5), or (6). With the calculated value of formula (1) satisfying the range of formula (4), the hardness increases when the calculated value of formula (1) is 10 or more. However, regeneration of a passivation coating occurs more prominently, and the pitting corrosion can be suppressed more effectively when the calculated values of formulae (2) and (3) satisfy the range of formula (4). This improves the sulfide stress corrosion cracking resistance.

The calculated value of formula (1) is preferably 5 to 45 in the following formula (4), and is preferably −5 to 5 in the following formula (5).

−109.37C+7.307Mn+6.399Cr+6.329Cu+11.343Ni−13.529Mo+1.276W+2.925Nb+196.775N−2.621Ti−120.307  Formula (1)

−0.0278Mn+0.0892Cr+0.00567Ni+0.153Mo−0.0219W−1.984N+0.208Ti−1.83  Formula (2)

−1.324C+0.0533Mn+0.0268Cr+0.0893Cu+0.00526Ni+0.0222Mo−0.0132W−0.473N−0.5Ti−0.514,  Formula (3)

In the formulae (1) to (3), C, Mn, Cr, Cu, Ni, Mo, W, Nb, N, and Ti represent the content of each element in mass % (the content is 0 (zero) percent for elements that are not contained)

−10≤formula (1)≤45, −0.25≤formula (2)≤−0.20, and 0.10≤formula (3)≤0.20  Formula (4)

−10≤formula (1)≤5, −0.355≤formula (2)≤−0.25, and 0.0255≤formula (3)≤0.10  Formula (5)

−10≤formula (1)≤−5, −0.39≤formula (2)≤−0.35, and −0.15≤formula (3)≤0.025  Formula (6)

In addition to the foregoing components, the composition contains the balance Fe and unavoidable impurities. The foregoing basic composition may further contain one or more selectable elements selected from Ti: 0.19% or less, Nb: 0.25% or less, W: 1.1% or less, and Co: 0.45% or less, as needed.

Titanium and niobium form carbides, and can reduce the solid-solution carbon. This makes it possible to reduce hardness. Excessively high Ti and Nb contents may deteriorate toughness, and the Ti and Nb contents are limited to 0.19% or less for Ti, and 0.25% or less for Nb when containing these elements.

Tungsten and cobalt are elements that improve the pitting corrosion resistance. However, excessively high W and Co contents may deteriorate toughness, and increase the material cost. For this reason, the W and Co contents are limited to 1.1% or less for W, and 0.45% or less for Co when containing these elements.

A preferred method for producing the martensitic stainless steel seamless pipe for oil country tubular goods according to aspects of the present invention is described below.

Aspects of the present invention use a steel pipe material of the composition described above. The method of production of the steel pipe material, or a seamless stainless steel pipe, is not particularly limited, and any known seamless steel pipe production method may be used.

Preferably, a molten steel of the foregoing composition is made into steel using a steel making process such as by using a converter furnace, and formed into a steel pipe material, for example, a billet, using a method such as continuous casting, and ingot casting-slab rolling. The steel pipe material is heated, and hot worked using a known pipe manufacturing process, for example, such as the Mannesmann-plug mill process, and the Mannesmann-mandrel mill process to produce a seamless steel pipe of the foregoing composition.

The process that follows the production of the steel pipe from the steel pipe material is not particularly limited. Preferably, the steel pipe is subjected to quenching, in which the steel pipe is heated to a temperature equal to or greater than the Ac₃ transformation point, and air cooled to a cooling stop temperature of 100° C. or less at a cooling rate of 0.1° C./s or more, and this is followed by tempering at a temperature equal to or less than the Act transformation point.

Quenching

In accordance with aspects of the present invention, the steel pipe is subjected to quenching, in which the steel pipe is reheated to a temperature equal to or greater than the Ac₃ transformation point, maintained for preferably at least 5 min, and air cooled to a cooling stop temperature of 100° C. or less. This produces a fine martensite phase, and high toughness. When the quenching heating temperature is less than the Ac₃ transformation point, heating cannot be made in the single austenite phase region, and a sufficient martensite structure cannot be obtained in the subsequent cooling. In this case, the desired high strength cannot be obtained. For this reason, the quenching heating temperature is limited to a temperature equal to or greater than the Ac₃ transformation point. Here, “air cooling” means a cooling rate of 0.1° C./s or more.

Tempering

Quenching of the steel pipe is followed by tempering. The tempering is a process by which the steel pipe is heated to a temperature equal to or less than the Ac₁ transformation point, maintained for preferably at least 10 min, and air cooled. When the tempering temperature is higher than the Ac₁ transformation point, the martensite phase precipitates after the tempering, and it is not possible to obtain the desired high toughness and excellent corrosion resistance. For this reason, the tempering temperature is limited to a temperature equal to or less than the Ac₁ transformation point. The Ac₃ transformation point (° C.), and the Ac₁ transformation point (° C.) can be measured by a Formaster test, in which the test piece is given a heating and cooling temperature history, and the transformation point is detected from a small displacement due to expansion and contraction.

EXAMPLES

Aspects of the present invention are further described below through Examples.

Molten steels of the compositions shown in Table 1 were made into steel with a converter furnace, and cast into billets (steel pipe material) by continuous casting. The steel pipe material was then hot worked with a model seamless rolling machine, and air cooled (cooling rate of 0.5° C./s) to produce a seamless steel pipe measuring 83.8 mm in outer diameter and 12.7 mm in wall thickness.

The seamless steel pipe was cut to obtain a test material, which was then subjected to quenching and tempering under the conditions shown in Table 2. A test piece for structure observation was collected from the quenched and tempered test material, and was polished, and measured for residual austenite (γ) amount by an X-ray diffraction method.

Specifically, the diffraction X-ray integral intensities of the γ (220) plane and the α (211) plane were measured. The results were then converted using the following equation.

γ(Volume fraction)=100/(1+(IαRγ/IγRα))

In the equation, Iα represents the integral intensity of α, Rα represents a crystallographic theoretical value for α, Iγ represents the integral intensity of γ, and Rγ represents a crystallographic theoretical value for γ.

A strip specimen specified by API standard 5CT was collected from the quenched and tempered test material, and subjected to a tensile test according to the API specifications to determine its tensile characteristics (yield stress YS, tensile stress TS). The Ac₃ point (° C.) and the Ac₁ point (° C.) shown in Table 2 were measured by conducting a Formaster test for a test piece (measuring 4 mm in diameter ϕ×10 mm) collected from the quenched test material. Specifically, the test piece was heated to 500° C. at 5° C./s, maintained for 10 minutes after raising the temperature to 920° C. at 0.25° C./s, and cooled to room temperature at 2° C./s. The Ac₃ point (° C.) and the Ac₁ point (° C.) were found by detecting the expansion and contraction of the test piece with the temperature history.

The SC test was conducted according to NACE TM0177, Method A. The test environment was created by using a 0.165 mass % NaCl test solution after adjusting the solution pH to 3.5 by addition of 0.41 g/L of CH₃COONa and HCl, and the test was conducted under a hydrogen sulfide partial pressure of 0.1 MPa, and an applied stress equal to 90% of the yield stress.

The results are presented in Table 2.

TABLE 1 Composition (mass %) Steel Ti, Nb, W, No. C Si Mn P S Cu Ni Cr Al Mo V N Co A 0.0088 0.205 0.11 0.013 0.0009 2.04 7.25 14.16 0.052 2.513 0.010 0.0471 Nb: 0.091 B 0.0075 0.200 0.45 0.015 0.0010 2.50 6.40 13.30 0.020 2.600 0.015 0.0085 Ti: 0.025, Nb: 0.08 C 0.0075 0.200 0.40 0.015 0.0010 2.40 6.25 13.20 0.020 2.550 0.015 0.0085 Ti: 0.05, Nb: 0.01 D 0.0075 0.200 0.40 0.015 0.0010 1.80 6.00 12.30 0.040 2.850 0.015 0.0075 Ti: 0.05, Nb: 0.005 E 0.0075 0.200 0.40 0.015 0.0010 0.40 5.70 12.00 0.040 2.550 0.015 0.0075 Ti: 0.05, Nb: 0.005 F 0.0075 0.200 0.40 0.015 0.0010 1.60 5.70 12.20 0.040 2.600 0.015 0.0075 Ti: 0.05, Nb: 0.005 G 0.0075 0.204 0.41 0.015 0.0009 2.40 6.24 13.22 0.020 2.553 0.015 0.0043 — H 0.0059 0.198 0.41 0.014 0.0010 1.81 5.99 12.32 0.042 2.850 0.014 0.0065 — I 0.0063 0.200 0.40 0.015 0.0009 0.41 5.71 12.02 0.043 2.552 0.014 0.0066 — J 0.0076 0.210 0.45 0.014 0.0011 2.51 6.40 13.30 0.020 2.605 0.015 0.0032 W: 0.16 K 0.0072 0.195 0.40 0.013 0.0010 0.39 5.71 12.00 0.042 2.550 0.016 0.0074 Co: 0.2 L 0.0069 0.189 0.42 0.014 0.0009 1.95 6.34 12.57 0.046 2.555 0.018 0.0070 — M 0.0080 0.192 0.11 0.013 0.0009 2.03 7.21 12.32 0.052 2.529 0.013 0.0067 — N 0.0060 0.195 0.10 0.012 0.0008 2.00 7.16 12.38 0.043 2.519 0.014 0.0064 — O 0.0064 0.216 0.21 0.018 0.0011 0.01 5.85 12.62 0.044 2.179 0.018 0.0056 — P 0.0072 0.223 0.20 0.014 0.0011 0.01 5.98 12.04 0.037 2.168 0.018 0.0076 — Composition (mass %) Steel Value of Value of Value of Applied No. formula (1) (*1) formula (2) (*2) formula (3) (*3) formula (*4) Remarks A 40.8 −0.238 0.114 (4) Compliant Example B 22.4 −0.234 0.155 (4) Compliant Example C 19.4 −0.245 0.126 (4) Compliant Example D 2.8 −0.278 0.054 (5) Compliant Example E −7.4 −0.353 −0.087 (6) Compliant Example F 0.8 −0.327 0.026 (5) Compliant Example G 18.7 −0.245 0.154 (4) Compliant Example H 3.0 −0.285 0.084 (5) Compliant Example I −7.0 −0.359 −0.059 (6) Compliant Example J 21.3 −0.231 0.168 (4) Compliant Example K −7.2 −0.363 −0.063 (6) Compliant Example L 13.5 −0.307 0.097 (4) Comparative Example M 20.2 −0.320 0.084 (4) Comparative Example N 20.0 −0.315 0.084 (4) Comparative Example O −0.7 −0.355 −0.096 (5) Comparative Example P −2.5 −0.411 −0.113 (6) Comparative Example The balance is Fe and unavoidable impurities (*1) Formula (1): −109.37C + 7.307Mn + 6.399Cr + 6.329Cu + 11.343Ni − 13.529Mo + 1.276W + 2.925Nb + 196.775N − 2.621Ti − 120.307 (*2) Formula (2): −0.0278Mn + 0.0892Cr + 0.00567Ni + 0.153Mo − 0.0219W − 1.984N + 0.208Ti − 1.83 (*3) Formula (3): −1.324C + 0.0533Mn + 0.0268Cr + 0.0893Cu + 0.00526Ni + 0.0222Mo − 0.0132W − 0.473N − 0.5Ti − 0.514 (*4) The formula used for determination Formula (4): −10 ≤ formula (1) ≤ 45, −0.25 ≤ formula (2) ≤ −0.20, and 0.10 ≤ formula (3) ≤ 0.20 Formula (5): −10 ≤ formula (1) ≤ 5, −0.35 ≤ formula (2) ≤ −0.25, and 0.025 ≤ formula (3) ≤ 0.10 Formula (6): −10 ≤ formula (1) ≤ −5, −0.39 ≤ formula (2) ≤ −0.35, and −0.15 ≤ formula (3) ≤ 0.025

TABLE 2 Tensile Quenching Tempering characteristics Heating Cooling Heating Structure Yield Tensile SSC Steel Ac₃ temper- Holding stop Ac₁ temper- Holding Residual stress stress resistance pipe Steel point ature time temperature point ature time γ (*1) YS TS test No. No. (° C.) (° C.) (min) Cooling (° C.) (° C.) (° C.) (min) (volume %) (MPa) (MPa) Cracking Remarks 1 A 750 920 20 Air 25 640 615 60 43.5 835 986 Absent Present cooling Example 2 B 750 920 20 Air 25 645 625 60 25.6 828 952 Absent Present cooling Example 3 C 755 920 20 Air 25 635 630 60 21.7 851 967 Absent Present cooling Example 4 D 750 920 20 Air 25 630 615 60 5.3 846 929 Absent Present cooling Example 5 E 755 920 20 Air 25 660 565 60 0.0 829 864 Absent Present cooling Example 6 F 745 920 20 Air 25 635 625 60 2.4 863 892 Absent Present cooling Example 7 G 755 920 20 Air 25 635 625 60 22.1 830 964 Absent Present cooling Example 8 H 750 920 20 Air 25 625 615 60 6.3 851 903 Absent Present cooling Example 9 I 755 920 20 Air 25 655 600 60 0.0 832 871 Absent Present cooling Example 10 J 750 920 20 Air 25 640 605 60 24.2 842 975 Absent Present cooling Example 11 K 750 920 20 Air 25 660 610 60 0.0 836 862 Absent Present cooling Example 12 L 755 990 20 Air 25 630 605 60 15.3 906 1009 Present Comparative cooling Example 13 M 720 920 20 Air 25 625 620 60 24.1 835 965 Present Comparative cooling Example 14 N 730 930 20 Air 25 620 600 60 23.5 859 958 Present Comparative cooling Example 15 O 740 920 20 Air 25 660 595 60 0.1 852 885 Present Comparative cooling Example 16 P 730 920 20 Air 25 630 590 60 0.0 847 880 Present Comparative cooling Example 17 A 750 730 20 Air 25 640 615 60 49.8 792 866 Present Comparative cooling Example 18 C 755 920 20 Air 25 635 650 60 34.6 772 811 Present Comparative cooling Example (*1) Residual γ: Residual austenite

The martensitic stainless steel seamless pipes of the present examples all had high strength with a yield stress of 758 MPa or more, and excellent SSC resistance that did not involve cracking even under the applied stress in the H₂S environment. Comparative Examples outside the range of the present invention did not show excellent SSC resistance, though the desired levels of high strength were obtained. 

1. A martensitic stainless steel seamless pipe for oil country tubular goods, the martensitic stainless steel seamless pipe having a composition that comprises, in mass %, C: 0.035% or less, Si: 0.5% or less, Mn: 0.05 to 0.5%, P: 0.03% or less, S: 0.005% or less, Cu: 2.6% or less, Ni: 5.3 to 7.3%, Cr: 11.8 to 14.5%, Al: 0.1% or less, Mo: 1.8 to 3.0%, V: 0.2% or less, N: 0.1% or less, and the balance Fe and unavoidable impurities, and that satisfies the following formula (4) with the following formulae (1), (2), and (3), the martensitic stainless steel seamless pipe having a yield stress of 758 MPa or more. −109.37C+7.307Mn+6.399Cr+6.329Cu+11.343Ni−13.529Mo+1.276W+2.925Nb+196.775N−2.621Ti−120.307  Formula (1) −0.0278Mn+0.0892Cr+0.00567Ni+0.153Mo−0.0219W−1.984N+0.208Ti−1.83   Formula (2) −1.324C+0.0533Mn+0.0268Cr+0.0893Cu+0.00526Ni+0.0222Mo−0.0132W−0.473N−0.5Ti−0.514,  Formula (3) In the formulae (1) to (3), C, Mn, Cr, Cu, Ni, Mo, W, Nb, N, and Ti represent the content of each element in mass % (the content being 0 (zero) percent for elements that are not contained). −10≤formula (1)≤45, −0.25≤formula (2)≤−0.20, and 0.10≤formula (3)≤0.20  Formula (4)
 2. The martensitic stainless steel seamless pipe for oil country tubular goods according to claim 1, wherein the composition further comprises, in mass %, at least one selected from Ti: 0.19% or less, Nb: 0.25% or less, W: 1.1% or less, and Co: 0.45% or less.
 3. A method for producing the martensitic stainless steel seamless pipe for oil country tubular goods of claim 1, the method comprising: making a steel pipe out of a steel pipe material; subjecting the steel pipe to quenching in which the steel pipe is heated to a temperature equal to or greater than the Ac₃ transformation point, and air cooled to a cooling stop temperature of 100° C. or less at a cooling rate of 0.1° C./s or more; and tempering the steel pipe at a temperature equal to or less than the Ac₁ transformation point.
 4. A method for producing the martensitic stainless steel seamless pipe for oil country tubular goods of claim 2, the method comprising: making a steel pipe out of a steel pipe material; subjecting the steel pipe to quenching in which the steel pipe is heated to a temperature equal to or greater than the Ac₃ transformation point, and air cooled to a cooling stop temperature of 100° C. or less at a cooling rate of 0.1° C./s or more; and tempering the steel pipe at a temperature equal to or less than the Ac₁ transformation point. 